Acoustic levitation of super-wavelength elastic films using ultrasound phased arrays

We present a method for levitating films with a surface size larger than the wavelength using airborne ultrasound phased arrays. A typical example is a polyimide film with a side length of 40–50 mm and a thickness of 5 μm (aspect ratio: 8–10 × 103). We verified our method by measuring the height, horizontal position, and vibration of the levitating film. The results show that the film levitates at the height of the original standing wave node and at discrete horizontal positions approximately every transducer interval. The levitated film vibrates at the same frequency as the ultrasonic transducer and cannot be regarded as rigid against ultrasonic waves. Different film materials and thicknesses were examined, including metal foils and wood papers. In this study, the maximum surface density of the films that levitated was 3.5 ×10−2 mg/mm2. Therefore, the proposed method can be used to hold film samples in the air for observation or as an aerial screen.